This invention relates to navigable vessels and one of its principal objects is to provide a vessel with improved performance, particularly a vessel that creates less turbulence, has less frictional resistance, and performs better in disturbed water and heavy seas, while maintaining stability and operating capability. The result of such improvements is increased fuel economy and/or speed, smoother operation, less structural demands on the vessel and improved operation in disturbed water.
A vessel moving through water experiences frictional resistance at the wetted surface below the water line. As the speed of the vessel increases the turbulence created by the hull moving through the water increases rapidly until frictional forces become the practical barrier to higher speed. The energy required to propel the vessel increases correspondingly. Improving speed and efficiency are recognized as the primary goals and activities in the naval arts and decreasing frictional resistance is seen as the key to these goals. An additional goal has been to improve the ability to operate in disturbed water, including heavy seas, where pitching and slamming, spray, yaw and roll severely limit navigability.
Another important factor in addition to speed that influences the efficiency of the vessel is its ability to maintain trim. Trim is the attitude at which the vessel is designed to lie when at rest. For displacement vessels this will usually be the same attitude that the vessel assumes at speed. It is important both for convenience and practicality that the decks, working, areas, equipment, etc. be at the same relationship to the horizontal (i.e. be horizontal) both when the vessel is at rest and when underway.
However, planing vessels normally are designed to (and must) operate at a positive trim angle, usually two to ten degrees, so that the stern remains in the water enough to maintain the stability of the vessel, not only against roll and yaw but also against pitching forces that could take the bow under, and to keep the propelling mechanisms submerged. (Loss of trim is usually measured as an angular deviation of the vessel's horizontal center for gravity line from true horizontal, zero degrees being perfect trim.) Such deviation from trim imposes a substantial penalty of increased friction and drag due to sinking of the stern, increased pitching, slamming and yaw, wind action against and air entrainment under the upraised bow and a substantial spray root at the bow entrance, as well as a decrease in the efficiency of the propelling system in most cases. Thus, maintaining trim is an additional objective in the art to further the primary goal of improving speed and efficiency.